The instant inventors, Dr. Silvia Pastorekova and Dr. Jaromir Pastorek, with Dr. Jan Zavada [“Zavada et al.”], discovered MN/CA IX, a cancer related cell surface protein originally named MN. [73, 123; Zavada et al., U.S. Pat. No. 5,387,676 (Feb. 7, 1995).] Zavada et al., WO 93/18152 (published 16 Sep. 1993) and Zavada et al., WO 95/34650 (published 21 Dec. 1995) disclosed the discovery of the MN gene and protein and the strong association of MN gene expression and tumorigenicity led to the creation of methods that are both diagnostic/prognostic and therapeutic for cancer and precancerous conditions. Zavada et al. disclosed further aspects of the MN/CA IX protein and the MN/CA9 gene in Zavada et al., WO 00/24913 (published 4 May 2000).
Zavada et al. cloned and sequenced the MN cDNA and gene, and revealed that MN belongs to a carbonic anhydrase family of enzymes that catalyze the reversible hydration of carbon dioxide to bicarbonate and proton [66, 72]. MN protein (renamed to carbonic anhydrase IX, CA IX) is composed of an extracellular part containing a N-terminal proteoglycan-like region and a catalytically active carbonic anhydrase domain. It is anchored in the plasma membrane by a single transmembrane region and a short intracytoplasmic tail.
Expression of CA IX is restricted to only few normal tissues [74], but is tightly associated with tumors [123]. It is also regulated by cell density in vitro [52] and is strongly induced by tumor hypoxia both in vitro and in vivo [121]. Numerous clinical papers describe the value of CA IX as an indicator of poor prognosis. All CA IX-related studies performed so far support the assumption made in the original Zavada et al., U.S. Pat. No. 5,387,676 that CA IX is useful as a diagnostic and/or prognostic tumor marker and as a therapeutic target.
MN/CA IX consists of an N-terminal proteoglycan-like domain that is unique among the CAs, a highly active CA catalytic domain, a single transmembrane region and a short intracytoplasmic tail [66, 72, 74, 116]. CA IX is particularly interesting for its ectopic expression in a multitude of carcinomas derived from cervix uteri, ovarian, kidney, lung, esophagus, breast, colon, endometrial, bladder, colorectal, prostate, among many other human carcinomas, contrasting with its restricted expression in normal tissues, namely in the epithelia of the gastrointestinal tract [8, 11, 21, 35, 41, 48, 50, 51, 56, 66, 72, 74, 86, 110, 111, 113, 116, 121, 122].
Uemura et al. [112] reported in 1997 that the G250 antigen was identical to MN/CA IX, years after MN/CA IX had been discovered and sequenced by Zavada et al. {[73, 123]; see also Pastorek et al. [72] and Opavsky et al. [66]}. Uemura et al. [112] stated: “Sequence analysis and database searching revealed that G250 antigen is identical to MN a human tumor-associated antigen identified in cervical carcinoma (Pastorek et al., 1994).”
MN/CA 9 and MN/CA IX—Sequence Similarities
FIG. 1A-C shows the full-length MN/CA9 cDNA sequence of 1522 base pairs (bps) [SEQ ID NO: 1], and the full-length MN/CA IX amino acid (aa) sequence of 459 aa [SEQ ID NO: 2]. FIG. 2A-F provides the 10,898 bp genomic sequence of MN/CA9 [SEQ ID NO: 3].
Computer analysis of the MN cDNA sequence was carried out using DNASIS and PROSIS (Pharmacia Software packages). GenBank, EMBL, Protein Identification Resource and SWISS-PROT databases were searched for all possible sequence similarities. In addition, a search for proteins sharing sequence similarities with MN was performed in the MIPS databank with the FastA program [75].
The proteoglycan-like domain [aa 53-111; SEQ ID NO: 4] which is between the signal peptide and the CA domain, shows significant homology (38% identity and 44% positivity) with a keratan sulphate attachment domain of a human large aggregating proteoglycan aggrecan [28].
The CA domain [aa 135-391; SEQ ID NO: 5] is spread over 265 aa and shows 38.9% amino acid identity with the human CA VI isoenzyme [5]. The homology between MN/CA IX and other isoenzymes is as follows: 35.2% with CA II in a 261 aa overlap [63], 31.8% with CA I in a 261 aa overlap [7], 31.6% with CA IV in a 266 aa overlap [65], and 30.5% with CA III in a 259 aa overlap [55].
In addition to the CA domain, MN/CA IX has acquired both N-terminal and C-terminal extensions that are unrelated to the other CA isoenzymes. The amino acid sequence of the C-terminal part, consisting of the transmembrane anchor and the intracytoplasmic tail, shows no significant homology to any known protein sequence.
The MN gene (MN/CA9 or CA9) was clearly found to be a novel sequence derived from the human genome. The overall sequence homology between the cDNA MN/CA9 sequence and cDNA sequences encoding different CA isoenzymes is in a homology range of 48-50% which is considered by ones in the art to be low. Therefore, the MN/CA9 cDNA sequence is not closely related to any CA cDNA sequences.
Very few normal tissues have been found to express MN protein to any significant degree. Those MN-expressing normal tissues include the human gastric mucosa and gallbladder epithelium, and some other normal tissues of the alimentary tract. Paradoxically, MN gene expression has been found to be lost or reduced in carcinomas and other preneoplastic/neoplastic diseases in some tissues that normally express MN, e.g., gastric mucosa.
CA IX, Hypoxia and Acidification of Extracellular Environment
Strong association between CA IX expression and intratumoral hypoxia (either measured by microelectrodes, or detected by incorporation of a hypoxic marker pimonidazole, or by evaluation of extent of necrosis) has been demonstrated in the cervical, breast, head and neck, bladder and non-small cell lung carcinomas (NSCLC) [8, 11, 21, 35, 48, 56, 111, 122]. Moreover, in NSCLC and breast carcinomas, correlation between CA IX and a constellation of proteins involved in angiogenesis, apoptosis inhibition and cell-cell adhesion disruption has been observed, possibly contributing to strong relationship of this enzyme to a poor clinical outcome [8]. Hypoxia is linked with acidification of extracellular milieu that facilitates tumor invasion and CA IX is believed to play a role in this process via its catalytic activity [86]. Thus, inhibition of MN/CA IX by specific inhibitors is considered to constitute a novel approach to the treatment of cancers in which CA IX is expressed.
Acidic extracellular pH (pHe) has been associated with tumor progression via multiple effects including up-regulation of angiogenic factors and proteases, increased invasion, and impaired immune functions [86, 124, 125, 130, 132]. In addition, it can influence the uptake of anticancer drugs and modulate the response of tumor cells to conventional therapy [86, 126]. Acidification of the tumor microenvironment was generally assigned to accumulation of lactic acid excessively produced by glycolysis and poorly removed by inadequate tumor vasculature. A high rate of glycolysis is especially important for hypoxic cells that largely depend on anaerobic metabolism for energy generation. However, experiments with glycolysis-deficient cells indicate that production of lactic acid is not the only mechanism leading to tumor acidity. The deficient cells produce only diminished amounts of lactic acid, but form acidic tumors in vivo [134, 144]. A comparison of the metabolic profiles of the glycolysis-impaired and parental cells revealed that CO2, in addition to lactic acid, is a significant source of acidity in tumors [127]. That data indicates that carbonic anhydrases could contribute to the acidification of the tumor microenvironment.
The CA IX isoform is identified herein as the best candidate for the role in acidifying the tumor microenvironment. First, CA IX is an integral plasma membrane protein with an extracellularly exposed enzyme active site [66, 72]. Second, CA IX has a very high catalytic activity with the highest proton transfer rate among the known CAs [116]. Third, CA IX is present in few normal tissues, but its ectopic expression is strongly associated with many frequently occurring tumors. Finally, CA IX level dramatically increases in response to hypoxia via a direct transcriptional activation of CA9 gene by HIF-1 [121], and its expression in tumors is a sign of poor prognosis [136]. Taken together, CA IX is herein considered to have all the qualities necessary to control tumor pH. That concept is supported by the proof provided herein that CA IX has the capacity to acidify extracellular pH.
CAIs
Teicher et al. [106] reported that acetazolamide—the prototypical CA inhibitor (CAI)—functions as a modulator in anticancer therapies, in combination with different cytotoxic agents, such as alkylating agents; nucleoside analogs; platinum derivatives, among other such agents, to suppress tumor metastasis and to reduce the invasive capacity of several renal carcinoma cell lines (Caki-1, Caki-2, ACHN, and A-498). Such studies demonstrate that CAIs may be used in the management of tumors that overexpress one or more CA isozymes. It was hypothesized that the anticancer effects of acetazolamide (alone or in combination with such drugs) might be due to the acidification of the intratumoral environment ensuing after CA inhibition, although other mechanisms of action of this drug were not excluded [20]. Chegwidden et al. 2001 hypothesized that the in vitro inhibition of growth in cell cultures, of human lymphoma cells with two other potent, clinically used sulfonamide CAIs, methazolamide and ethoxzolamide, is probably due to a reduced provision of bicarbonate for nucleotide synthesis (HCO3− is the substrate of carbamoyl phosphate synthetase II) as a consequence of CA inhibition [20].
All the six classical CAIs (acetazolamide, methazolamide, ethoxzolamide, dichlorophenamide, dorzolamide, and dichlorophenamide) used in clinical medicine or as diagnostic tools, show some tumor growth inhibitory properties [18, 78, 101, 102].
The inventors, Dr. Claudia Supuran and Dr. Andrea Scozzafava, reported the design and in vitro antitumor activity of several classes of sulfonamide CAIs, shown to act as nanomolar inhibitors against the classical isozymes known to possess critical physiological roles, such as CA I, CA II and CA IV. Those compounds were also shown to exert potent inhibition of cell growth in several leukemia, non-small cell lung, ovarian, melanoma, colon, CNS, renal, prostate and breast cancer cell lines, with GI50 values of 10-75 nM in some cases [77, 91, 92, 100].
Wingo et al. reported that three classic sulfonamide drugs (acetozolamide, ethoxzolamide and methoxzolamide) inhibited CA IX carbonic anhydrase activity with values of KI in the nanomolar range [116]. However, until the present invention, no systematic structure-activity relationship study of sulfonamide inhibition of CA IX, alone or in comparison to other CA isozymes had been performed.
Certain pyridinium derivatives of aromatic/heterocyclic sulfonamides have shown nanomolar affinities both for CA II, as well as CA IV, and more importantly, they were unable to cross the plasma membranes in vivo [17].
Sterling et al. [85] investigated the functional and physical relationship between the downregulated in adenoma bicarbonate transporter and CA II, by using membrane-impermeant sulfonamide inhibitors (in addition to the classical inhibitors such as acetazolamide), which could clearly discriminate between the contribution of the cytosolic and membrane-associated isozymes in these physiological processes.
CAS
Carbonic anhydrases (CAs) form a large family of genes encoding zinc metalloenzymes of great physiological importance. As catalysts of reversible hydration of carbon dioxide, these enzymes participate in a variety of biological processes, including respiration, calcification, acid-base balance, bone resorption, formation of aqueous humor, cerebrospinal fluid, saliva and gastric acid [reviewed in Dodgson et al. (27)]. CAs are widely distributed in different living organisms. In higher vertebrates, including humans, 14 different CA isozymes or CA-related proteins (CARP) have been described, with very different subcellular localization and tissue distribution [40, 93, 95, 94, 102]. Basically, there are several cytosolic forms (CA I-III, CA VII), four membrane-bound isozymes (CA IV, CA IX, CA XII and CA XIV), one mitochondrial form (CA V) as well as a secreted CA isozyme, CA VI [40, 93, 94, 95, 102].
It has been shown that some tumor cells predominantly express only some membrane-associated CA isozymes, such as CA IX and CA XII [2, 67, 68, 78, 87, 93, 95]. Occasionally, nuclear localization of some isoenzymes has been noted [64, 69, 70]. Not much is presently known about the cellular localization of the other isozymes.
CAs and CA-related proteins show extensive diversity in their tissue distribution, levels, and putative or established biological functions [105]. Some of the CAs are expressed in almost all tissues (CA II), while the expression of others appears to be more restricted (e.g., CA VI and CA VII in salivary glands [32, 69, 71]. The CAs and CA-related proteins also differ in kinetic properties and susceptibility to inhibitors [82].
Most of the clinically used sulfonamides mentioned above are systemically acting inhibitors showing several undesired side effects due to inhibition of many of the different CA isozymes present in the target tissue/organ (14 isoforms are presently known in humans) [93, 94, 95, 102]. Therefore, many attempts to design and synthesize new sulfonamides were recently reported, in order to avoid such side effects [13, 17, 42, 62, 80, 99, 100]. At least four CA isozymes (CA IV, CA IX, CA XII and CA XIV) are associated to cell membranes, with the enzyme active site generally oriented extracellularly [93, 94, 95, 102]. Some of these isozymes were shown to play pivotal physiological roles (such as for example CA IV and XII in the eye, lungs and kidneys, CA IX in the gastric mucosa and many tumor cells) [3, 18, 22, 29, 49, 67, 68, 83, 93, 94, 95, 102], whereas the function of other such isozymes (CA XIV) is for the moment less well understood [93, 95]. Due to the extracellular location of these isozymes, if membrane-impermeant CA inhibitors (CAIs) could be designed, only membrane-associated CAs would be affected.
The first approach towards introducing the membrane-impermeability to CAIs from the historical point of view was that of attaching aromatic/heterocyclic sulfonamides to polymers, such as polyethyleneglycol, aminoethyldextran, or dextran [39, 60, 107]. Such compounds, possessing molecular weights in the range of 3.5-99 kDa, prepared in that way, showed indeed membrane-impermeability due to their high molecular weights, and selectively inhibited in vivo only CA IV and not the cytosolic isozymes (primarily CA II), being used in several renal and pulmonary physiological studies [39, 60, 107]. Due to their macromolecular nature, such inhibitors could not be developed as drugs/diagnostic tools, since in vivo they induced potent allergic reactions [39, 60, 93, 95, 107]. A second approach for achieving membrane-impermeability is that of using highly polar, salt-like compounds. Only one such sulfonamide has until recently been used in physiological studies, QAS (quaternary ammonium sulphanilamide), which has been reported to inhibit only extracellular CAs in a variety of arthropods (such as the crab Callinectes sapidus) and fish [57]. The main draw-back of QAS is its high toxicity in higher vertebrates [57].
Enzyme activity of carbonic anhydrases (including that of CA IX) can be efficiently blocked by sulfonamide inhibitors. That fact has been therapeutically exploited in diseases caused by excessive activities of certain CA isoforms (e.g. CA II in glaucoma). There is also an experimental evidence that sulfonamides may block tumor cell proliferation and invasion in vitro and tumor growth in vivo, but the targets of those sulfonamides have not been identified yet. However, the sulfonamides available so far indiscriminately inhibit various CA isoenzymes (14 are presently known in humans) that are localized in different subcellular compartments and play diverse biological roles. This lack of selectivity compromises the clinical utilization of these compounds (due to undesired side effects caused by concurrent inhibition of many CA isoforms) and represents a main drawback also for the sulfonamide application against CA IX in anticancer therapy.
Thus, there is a need in the art for membrane-impermeant, potent CA IX inhibitors, which would become doubly selective inhibitors for CA IX. The inventors have previously made and described some of the membrane-impermeant molecules described here; however, they were characterized only for their ability to inhibit CA I, CA II and CA IV. While others have studied effects of selective inhibition of extracellular CA by membrane impermeant agents in retinal prigmented epithelia or muscle [34, 120], these agents have not been characterized for their ability to inhibit CA IX. Since CA IX is one of the few extracellular carbonic anhydrases, a membrane-impermeant selective inhibitor of CA IX would be doubly selective for this enzyme and thereby avoid side effects associated with nonspecific CA inhibition.